State reasons for the following:

(i) pK_b value for aniline is more than that for methylamine.

(ii) Ethylamine is soluble in water whereas aniline is not soluble in water.

(iii) Primary amines have higher boiling points than tertiary amines.

(i) Basicity of a compound is determined by its tendency to donate more electrons. If a compound is more electron rich, its tendency to donate electrons would be more so its basic strength would be more.

If groups showing electron pushing tendency (alkyl groups), are attached to the N atom, electron concentration would be more. Similarly more is the number of such groups, more is the basic strength.

In aromatic amines, the lone pair on nitrogen atom is delocalised all over the ring. Due to which they are not available for donation. So their basic strength is less than that of aromatic amines. If basic strength is less then its pK_b value is more.

Thus, pK_b value for aniline is more than that for methylamine.

(ii) Due to the large hydrocarbon part present in aniline, the nitrogen atom cannot form intermolecular hydrogen bonding with water due to which its solubility in water is less. Also the lone pair on nitrogen atom is delocalised all over the ring. Due to which they are not available for donation.

Whereas, the hydrocarbon part in methyl amine is less and compact so the nitrogen atom can take part in intermolecular hydrogen bonding with water. So, its solubility in water is more.

(iii) In primary amines, two H-atoms are attached to N-atom, so they can participate in intermolecular hydrogen bonding with water due to which they become highly stable. But tertiary amines, due to the absence of H-atom on N-atom cannot participate in intermolecular hydrogen bonding with water. Since primary amines are more stable, more energy would be required to break their stability. Hence, they have high boiling points.